1. Field of the Invention
The present invention relates to a method of purifying a polyacrylonitrile obtained by aqueous suspension polymerization by the use of an inorganic redox initiator; a process for producing acrylic fiber having a high spinning stability by using a purified solution of said polyacrylonitrile as a starting material; and a process for producing carbon fiber by carbonizing this acrylic fiber.
2. Description of the Related Art
Acrylic fibers have properties such as excellent bulking power, feeling and color definition like those of wool, and are utilized for various purposes The acrylic fiber is obtained as a product, i.e., staples or filaments, by subjecting a polyacrylonitrile as starting material to a step of its dissolution in an organic or inorganic solvent, and then spinning the resulting solution by a wet process, a dry process or a semi-dry semi-wet process. In general, the starting polyacrylonitrile is produced by radical polymerization of acrylonitrile monomer and a monomer copolymerizable therewith. The acrylic fiber obtained by the above method is carbonized into carbon fiber through a carbonizing step, and the carbon fiber has high performance characteristics such as high strength, modulus of elasticity and heat resistance and is utilized for various purposes, for example, it is used as a material for airplanes.
Suspension polymerization which is nonuniform polymerization in an aqueous system is a historied and widely used process for producing a polyacrylonitrile and is advantageos, for example, in that control of the quality of the polymer is easy, that recovery of an untreated monomer(s) is easy, and that the control of the whole process is easy. For the production of the polyacrylonitrile, a continuous suspension polymerization method using water as a medium for reaction is employed. As a polymerization initiator in this case, an inorganic initiator is generally used.
When monomers the greater part of which is acrylonitrile monomer are polymerized using an inorganic initiator (for example, an oxidation-reduction system of ammonium persulfate-ammonium hydrogensulfite-ferrous sulfate) and water acidified with sulfuric acid, as a medium for reaction, the resulting polyacrylonitrile forms particles, namely, the polyacrylonitrile is obtained in the form of an aqueous dispersion. The polyacrylonitrile usable as a material for acrylic fiber can be obtained by filtering the aqueous dispersion of the polymer after completion of the polymerization and washing and drying the precipitate. In this case, when an initiator containing iron is used, the resulting polyacrylonitrile contains iron compounds or iron ions.
For obtaining a polyacrylonitrile by continuous aqueous suspension polymerization, an aluminum reactor is absolutely necessary. This is because when a stainless reactor or a glass-lined reactor is used for producing a polyacrylonitrile by continuous aqueous suspension polymeization, the continuous polymerization becomes substantially difficult owing to the formation of a deposit of the polyacrylonitrile. It is said that when an aluminum reactor is used, its aluminum surface is corroded and dissolved because the reaction system is an acidic aqueous solution, whereby the formation of a deposit of the polyacrylonitrile is prevented.
For completing the polymerization reaction, a short-stop is used. The short-stop used for producing a polyacrylonitrile by aqueous suspension polymerization should have an ability to neutralize the acidic aqueous solution as reaction system. As the short-stop, there are used aqueous solutions of electrolytes such as sodium oxalate, ethylenediaminetetraacetic acid disodium salt, sodium hydrogencarbonate, etc.
Thus, a polyacrylonitrile is produced in the presence of at least electrolytes due to the polymerization initiator, electrolytes produced by the corrosion of a polymerizer, and the electrolytes added as short-stop. Moreover, with the progress of the polymerization reaction, the polyacrylonitrile forms polymer particles of several tens microns as dispersed particles in the aqueous system. Therefore, the polymer particles are contaminated with the above-mentioned various electrolytes such as iron compounds, aluminum compounds, etc.
In the production of an acrylic fiber from a polyacrylonitrile produced by such conventional aqueous suspension polymeization, the polymer obtained by the polymerization is subjected to a washing step and then a drying step, and dissolved in a solvent capable of dissolving the polyacrylonitrile, to obtain a polyacrylonitrile dope solution, which is spun by a wet process or a dry process to produce the acrylic fiber. Conventional polyacrylonitrile dope solutions, however, cause various problems in the production of recent high-quality acrylic fiber. For example, when the dope solution used for producing an acrylic fiber is held with heating (at about 100.degree. C.) for a long period of time, its yellowing becomes marked, and the dope solution itself is gelatinized, so that the flowability of the solution is deteriorated. The yellowing of the dope solution causes yellowing of the acrylic fiber itself The gelatinization of the dope solution results in a low flowability of the solution, so that stable spinnable properties cannot be assured. Consequently, end breakage is caused and the production of acrylic fiber having a sufficiently high quality becomes difficult. In addition, the above-mentioned various electrolytes remain in carbon fiber obtained by firing the acry ic fiber and deteriorate the strength and the quality. Thus, the aforesaid various electrolytes, in particular, the iron compounds or ions thereof and the aluminum compounds and ions thereof, have an undesirable influence on the thermal stability of the polyacrylonitrile dope solution, the spinnable properties at the time of spinning into fiber, and physical properties of the fiber.